P0308 on 2006 GMC Envoy XL: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2006 Gmc ENVOY XL

This code is only possible on the Envoy XL models equipped with the 5.3L V8 engine, as the standard 4.2L inline-6 engine does not have a cylinder 8. The 2006 Envoy's 5.3L V8 (RPO code LH6) features Active Fuel Management (AFM), a system designed to improve fuel economy by deactivating certain cylinders under light load. While AFM lifter failure is a notorious issue on these engines, it's important to note that cylinder 8 is NOT one of the AFM-deactivated cylinders. Therefore, a P0308 code should be diagnosed as a standard ignition or fuel issue first. However, a widespread engine problem (like low oil pressure) related to the AFM system can still manifest in non-AFM cylinders. Technical Service Bulletin #PIP4138L (and its successor, #19-NA-219) points to worn camshaft lobes and/or lifters as a potential cause for P030x codes if standard diagnostics don't find the issue.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough idle or engine shaking
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Engine may be hard to start
• Reduced fuel economy
• Smell of unburnt fuel from the exhaust.
• Engine ticking or knocking noise, which may point to a valvetrain issue.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Cylinder 8 is located at the rear of the engine bay where heat can be more concentrated, potentially leading to a higher failure rate for the coil.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, which is the next one forward on the passenger side). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty.
   Typical fix: Replace the faulty ignition coil.
   Est. part cost: $40 - $80
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, fouling (black, oily, or white deposits), or an incorrect gap. Compare it to a new plug. Oil fouling could indicate a leaking valve cover gasket.
   Typical fix: Replace the spark plug. 🎬 Watch: How to change the cylinder 8 spark plug. It is often recommended to replace all 8 plugs at the same time for consistent performance.
   Est. part cost: $10 - $25 per plug
3. Bad Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug The 5.3L V8 engine uses spark plug wires, which are a wear item. The insulation can break down over time, causing the spark to arc to the engine block instead of reaching the plug. A known issue on similar GM V8s is a coolant leak from the heater core hoses dripping directly onto the #8 spark plug wire, causing a misfire.
   How to confirm: Inspect the wire for any visible damage, cracks, or burn marks. Check for a secure 'click' at both the coil and plug. With the engine running in the dark, you can sometimes see the spark arcing from the wire to a metal surface.
   Typical fix: Replace the spark plug wire. It is best practice to replace the entire set of wires.
   Est. part cost: $50 - $100 for a set
4. Clogged or Defective Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Swap the fuel injector from cylinder 8 with another cylinder. If the misfire code follows the injector, it is the cause. This is more labor-intensive than swapping a coil. Alternatively, listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound similar to its neighbors.
   Typical fix: Replace the faulty fuel injector. Some owners have reported success clearing an intermittent misfire by using a high-quality fuel system cleaner like Techron over a couple of fuel tanks.
   Est. part cost: $60 - $150

Rare But Worth Checking

• Worn Camshaft Lobe and/or Failed Lifter: → Shop Engine Camshaft This is a known issue documented in manufacturer TSB #19-NA-219 (which supersedes #PIP4138L). It often presents with a distinct ticking, chirping, or squeaking noise from the engine that increases with RPM. A failed lifter roller can grind down the camshaft lobe, requiring replacement of the cam, lifters, and related components.
• Low Engine Compression: Issues like a burnt valve, leaking head gasket, broken valve spring, or worn piston rings can cause a loss of compression in cylinder 8, preventing proper combustion. A compression test and cylinder leak-down test are needed to diagnose this.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak from a failed intake manifold gasket near the cylinder 8 runner can cause the air/fuel mixture to become too lean, resulting in a misfire. This may be accompanied by a hissing sound or lean codes like P0171/P0174.
• Leaking Valve Cover Gasket: → Shop Engine Valve Cover A leaking valve cover gasket can allow engine oil to pool in the spark plug well for cylinder 8. This can saturate the spark plug boot and foul the plug, causing a persistent misfire.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308.
2. Check if the Check Engine Light is steady or flashing. A flashing light indicates a severe misfire that can damage the catalytic converter.
3. Visually inspect the area around cylinder 8 (passenger side, rear). Look for obvious issues like a loose plug wire, damaged wiring, or fluid leaks from above, paying close attention to the heater hoses.
🎬 See how common heater hose leaks cause this misfire.
4. Swap the ignition coil from cylinder 8 with cylinder 6. Clear the codes and drive. If the code changes to P0306, the coil is bad.
5. If the misfire remains on cylinder 8, remove and inspect the spark plug for wear, damage, or fouling. Check for oil in the spark plug tube, which indicates a bad valve cover gasket. Replace the plug if necessary.
6. Inspect the spark plug wire for cylinder 8 for any signs of damage, arcing, or a loose connection at the coil or plug.
7. If ignition components are good, listen for a distinct ticking sound from the engine, which could indicate a lifter/camshaft issue as described in TSB #19-NA-219.
8. If no obvious cause is found, more advanced diagnostics are needed, such as testing the fuel injector, performing an engine compression test, and checking for vacuum leaks near the intake manifold for cylinder 8.

Parts You'll Likely Need

• Ignition Coil (OEM #12699382) — This is one of the most frequent failure points for a single-cylinder misfire. The coil for cylinder 8 is at the back of the engine and subject to significant heat.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $60 - $100
  Aftermarket price range: $40 - $80
• Spark Plug (OEM #41-110) — Spark plugs are a routine maintenance item and a common cause of misfires as they wear out. The OEM recommended plug is an ACDelco Iridium plug.
  Trusted brands: ACDelco, NGK
  OEM price range: $15 - $25
  Aftermarket price range: $8 - $15
• Spark Plug Wire Set (OEM #19417612) — The 5.3L V8 uses spark plug wires, which can degrade over time and cause misfires. It's best to replace them as a set.
  Trusted brands: ACDelco, NGK, Delphi
  OEM price range: $80 - $120
  Aftermarket price range: $50 - $100

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) code may appear alongside P0308.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138L: Directly links P0300-P0308 codes to potential worn camshaft lobes or lifters if standard diagnostics fail. This TSB has been superseded.
• 19-NA-219: Replaces PIP4138L and provides detailed diagnostic information for engine noises and misfires (P0300-P0308) related to worn cam/lifters, sticking valves, and broken valve springs.
• PIP5628C: Communicates that diagnosing P0300-P0308 codes may require providing specific information to the GM Technical Assistance Center (TAC), indicating persistent diagnostic challenges.

Platform-Specific Known Issues

• As per TSB #19-NA-219 (superseding PIP4138L), if basic diagnostics do not resolve the misfire, technicians are advised to check for a worn camshaft lobe and/or lifter, which is a known mechanical failure point on this engine platform.
• A real-world failure pattern reported by owners of similar 5.3L V8s involves a coolant leak from the heater core quick-connect fittings located on the firewall, which can drip directly onto the cylinder 8 spark plug wire and coil, causing a misfire.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key ON, Engine OFF) — expected: 55-62 psi (385-425 kPa). Failure: Pressure below 55 psi can indicate a weak fuel pump or faulty regulator, leading to lean misfires across multiple cylinders.
• Ignition Coil Primary Resistance — expected: 0.4 - 0.6 ohms. Failure: A reading outside this range (especially an open circuit/infinite resistance) indicates a faulty primary winding.
• Ignition Coil Secondary Resistance — expected: 5,000 - 7,200 ohms. Failure: A reading significantly outside this range points to a breakdown in the secondary coil windings.
• Fuel Injector Resistance — expected: 12.0 - 12.5 ohms. Failure: Low resistance suggests a shorted injector coil, while infinite resistance indicates an open circuit.
• Running Compression Test (at idle) — expected: 60 - 90 PSI (should be consistent with a known good cylinder). Failure: Significantly lower pressure compared to other cylinders points towards a mechanical issue like a collapsed lifter, worn camshaft lobe, or poor valve sealing that may not be apparent in a static cranking compression test.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $A9: Provides access to the raw misfire counter data for cylinder 8 before it's severe enough to set a persistent P0308 code. It tracks both EWMA (Exponentially Weighted Moving Average) misfire counts over the last 10 drive cycles and misfire counts for the current drive cycle. (see via A professional scan tool or an advanced OBD-II app that can display Mode $06 data. A high count on TID $A9 relative to other cylinders (TID $A2-$A8) confirms cylinder 8 is the source of an intermittent issue.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2 or equivalent professional scanner: Cylinder Power Balance Test — To actively test the contribution of cylinder 8. The tool disables the fuel injector for each cylinder one by one. A healthy cylinder will cause a noticeable drop in RPM. If disabling cylinder 8 causes little or no RPM change, it confirms it is not contributing power.
• GM Tech 2 / GDS2 or equivalent professional scanner: Fuel Injector Balance Test — To test for a clogged or failing fuel injector after ignition issues are ruled out. With the engine off and fuel system pressurized, the tool pulses the injector for a set duration and measures the resulting drop in fuel pressure. The pressure drop for cylinder 8 should be within 1-2 PSI of the other cylinders.

Wiring & Ground Locations

• G103 — On the front of the passenger side (right) cylinder head.. This is a main engine wiring harness ground. A GM service bulletin identifies that corrosion at this specific ground can cause unexplained misfires on this engine family, including P0308. It directly affects components on the passenger side bank, including the coil and injector for cylinder 8.
• Cylinder 8 Injector Control — PCM Connector C1 (Blue), Pin 12. Wire color is typically Black/White.. This is the specific wire from the PCM that provides the ground signal to fire the fuel injector for cylinder 8. A continuity or voltage drop test on this wire can identify a wiring fault between the PCM and the injector.
• Cylinder 8 Ignition Control — PCM Connector C2 (Green), Pin 34. Wire color is typically Dark Green/White.. This is the specific wire from the PCM that sends the trigger signal to the ignition coil for cylinder 8. Probing this wire with an oscilloscope can verify if the PCM is commanding the coil to fire.
• Firewall Ground Strap — A braided strap connecting the rear of the passenger side cylinder head to the firewall.. Forum users report that a poor connection at this ground, located physically close to cylinder 8, can cause a variety of electrical issues, including crank-no-start or intermittent misfires.

Real Owner Repair Stories

• Reddit user in r/ChevyTrucks (2011 Suburban with 5.3L V8 (similar engine and layout)) — P0308 code.
  ❌ Tried (didn't work) Initial diagnosis pointed towards common ignition parts.
  ✅ What actually fixed it The root cause was a coolant leak from the heater core quick-connect fittings on the firewall. The coolant was dripping directly onto the #8 spark plug wire. The final fix was repairing the coolant leak and cleaning up the plug wire; the P0308 code was resolved without replacing ignition parts.

"I Checked Everything" — The Actual Cause

• A common diagnostic step for misfires is a smoke test to find vacuum leaks. However, if the smoke test is clean, the cause may be non-vacuum related. A corroded main engine ground (G103) on the passenger cylinder head is a documented cause of misfires that would not be found with a smoke test. Similarly, a coolant leak from the heater core hoses dripping onto the cylinder 8 ignition components is another real-world cause that is easily missed by standard diagnostic procedures.

OEM Part Supersession History

• 12570616, 12573190, 12611424 → 12699382 — Standard part evolution for improved durability, winding protection, and corrosion resistance.
  Heads up: The 12699382 coil is the correct 'round' style coil for this application. Ensure you are not trying to use an older 'flat' style coil from a different LS engine family.